As an electrode configuring a sensor electrode or a heater electrode for various gas sensors such as an oxygen sensor, a NOx sensor, and an exhaust gas temperature sensor, one obtained by calcination of a metal paste has been used hitherto. The reason why a metal paste is used in producing of these electrodes is that the metal paste can cope with a complicated pattern of an electrode, and further, that applying and calcining the metal paste on a green sheet forming a ceramic substrate allows a substrate and an electrode to be produced simultaneously, which is preferable from the viewpoint of production efficiency.
As the configuration of the metal paste for forming an electrode, a mixture of a conductive particle such as a precious metal and a ceramic powder such as Al2O3 and ZrO2 with a solvent is known. The reason for mixing a ceramic powder with a metal paste is to improve adhesion of an electrode, when the metal paste is applied and calcined on a green sheet to simultaneously produce a substrate and an electrode as described above, with a difference in shrinkage rate between the metal paste and the green sheet corrected to solve problems for warpage and deformation of the substrate. In addition, the ceramic powder secures moldability of an electrode film. On the other hand, there is also a disadvantage in that the ceramic powder increases a resistance value of an electrode film to be produced and is likely to make the resistance value higher than in an electrode of a bulk metal. Therefore, regarding the use of the ceramic powder, searching for an optimal usage type or mixing amount is a matter to be considered while taking into account a balance between the securing of moldability and the reducing of resistance.
Regarding the matter to be considered, the present inventors disclose a metal paste from which an electrode film of low resistance can be produced and which is excellent in adhesion and conformability to a substrate, and an electrode produced by using the same (Patent Document 1). In the metal paste suggested by the present inventors, as the configuration of a conductive particle, a conductive particle having a core/shell structure is used in which a ceramic powder is bonded and coated on an outer surface of a core particle including a precious metal. Further, when the conductive particle has a core/shell structure, an electrode of low resistance is formed by dispersion of the ceramic powder in a fine state in a process of sintering the metal paste and suppression of the coarsening of the ceramic powder that is a factor of increasing the resistance.